All You should Know about Brewery Yeast

18 February 2011

Yeast is probably the most important item used in the production of ale and beer, yet its significance is often underrated. But the lack of information gives the impression that yeast is of minor importance and that any old yeast will do the job. Unfortunately, this is not the case. It is not generally appreciated by the amateur brewer that about half of the components that go towards the flavour of beer are supplied by the yeast in the form of fermentation by-products. The quality, flavour, and aroma of a beer are all affected by the type and quality of the yeast employed.

Commercial breweries attach considerable importance to the quality and pedigree of their yeast strain. Each brewery uses its own strain which has been bred, nurtured, pampered and coaxed over a period of many years to produce the type and character of ale which best suits its customers. If a brewery were forced to changed its yeast strain, the chances are that it would result in a considerable change in the character of its beer. This could result in the brewery's established customers indignantly refusing to drink its product, even though it may still be perfectly good beer.

Little animicules in your wort!

Yeast is a form of fungus, a single-celled plant which has the ability to convert the sugars incur wort into roughly equal weights of alcohol and carbon dioxide. There are hundreds of different strains of yeast, many of which are quite unsuitable for beer making, but of those that are suitable, each individual strain of yeast differs in its characteristics and will impart different qualities to the finished beer; some desirable, some undesirable. The various strains of yeast also differ in other ways; such as the speed at which they ferment the wort, their ability to separate from the beer when fermentation is complete, their sugar tolerance, their alcohol tolerance and one hundred and one other things.

All brewers' yeasts fall into one of two broad classifications: top working, whereby the yeast rises to the surface of the wort during fermentation; and bottom working, whereby the yeast falls to the bottom of the vat during fermentation.

Top-working yeasts, saccharomyces cerevisae, often referred to as ale yeasts, are those where most of the yeast rises to the surface of the wort during fermentation. It is the type of yeast used in all British-style beers, and, incidentally, most of the beers of Europe.

Bottom-working yeasts, saccharomyces uvarum (formerly known as saccharomyces carlsbergenis), often referred to in Britain as lager yeasts, are those where most of the yeast settles to the bottom, of the vessel during fermentation, although this effect may not be evident in the home-brewing environment. The major characteristic of this type of yeast is that it will work at much lower temperatures than its top-working counterpart. It is used for Pilsner style lagers.

Heavy breathers!

Our pet yeast has two methods of respiration, aerobic (with oxygen), and anaerobic (without oxygen). Not all yeasts have the ability to respire by two different methods, but all brewers' yeasts must have this ability. Yeasts which are able to change from one form of respiration to another are known as facultative anaerobes. A brewers' yeast prefers to respire aerobically if given the opportunity, and will do so if the medium in which it is working has a plentiful supply of dissolved oxygen. However, when the supply of dissolved oxygen is used up, the yeast will revert to anaerobic respiration whereby it synthesises its oxygen (in fact, not strictly true) from the medium in which it is working.

When the yeast is first pitched it respires aerobically Utilising the dissolved oxygen in the wort. During this aerobic phase the yeast multiplies at an exponential rate and very quickly establishes itself. To ensure that this preliminary rapid growth phase takes place the brewer must ensure that the wort is well-aerated before the yeast is pitched, because the yeast will only multiply significantly if it is permitted to respire aerobically.

When the dissolved oxygen in the wort has been used up, the yeast is then forced to revert to anaerobic respiration. Reproduction noticeably slows down as the yeast adapts itself to anaerobic respiration. The yeast always finds it a bit of a struggle to respire anaerobically, but it is only under anaerobic conditions that alcohol is produced.

This brings us to an important point that is often not made clear in home-brewing books or in the instructions supplied with kits:

At the time that the yeast is added (pitched) it is important that the wort is well aerated, but from that point on it is important that care is taken to ensure that a minimum of air ever comes into contact with the beer again.

The boiling phase of ale production drives off any dissolved air that may be present. It is there-fore nessary to aerate the wort after it has cooled. Wort aeration is covered in the chapter on fermentation.

Yeast reproduction

Not only does brewers' yeast have two methods of respiration, but the crafty little blighters also have two methods of reproduction. The first of these methods is called budding and is similar to, but not the same as, simple cell division. Firstly a bud grows out of the side of the yeast cell, followed by the nucleus of the cell dividing into two, one half of the nucleus then goes into the bud which continues to grow until it is about half the size of the parent cell, at which point it separates from it. Brewers' yeasts do not reproduce by simple cell division (although some other yeasts do), where firstly the nucleus of the cell divides into two, followed by the whole cell dividing into two, with one half of the nucleus in each of the daughter cells.

The second method by which brewers' yeast reproduces is a sexual method whereby two yeast cells merge into one (fusion), followed some time later by the cell dividing up into four small spores, the cell usually continues to reproduce by budding for some considerable time between fusion and sporulation, but eventually the spores are thrown. These spores are particularly hardy and can survive conditions that a full-grown cell cannot. In times of hardship a yeast culture will sporulate as part of its survival mechanism, the resulting spores can remain inert for considerable periods of time.

Yeast nutrients

Yeast requires a similar diet to man. Sugars, proteins, mineral salts and vitamins are all necessary for nutrition. The sugars are broken down into alcohol and carbon dioxide during fermentation. Amino-acids and peptides are required, so are nitrogen, phosphates and trace elements of calcium and magnesium. Yeast, strangely enough, also requires vitamin B1. Some of the trace elements and supplements are used by the yeast cell directly, while others play a catalytic role (known as co-enzymes) in conjunction with the primary enzymes during the alcoholic reduction process. Sufficient trace elements will almost always be present in your wort. They are sup-plied by both the brewing water and the malt. It is possible although very rare, to produce a wort that is deficient in nitrogen or phosphates. This is normally due to a poor recipe (too much sugar) or to over-enthusiastic water treatment (too much calcium sulphate added).

Sucrose (Cane or beet sugar) does not contribute vitamins or minerals to the wort. In fact, domestic sugar is pure energy - nothing else. Unlike malt it does not supply the additional ingredients necessary to enable the yeast (or us for that matter) to convert its energy into a useful form. These additional ingredients have to be supplied by the malt. The amount of cane sugar added to a recipe should never exceed twenty per cent, preferably much less if problems with yeast performance are to be avoided.

Most beer kits, and many published recipes, specify far too much added cane sugar, sometimes in excess of 50 per cent, which is almost guaranteed the leave the yeast with a deficiency of nutrients. Although it is possible to remedy this problem by adding synthetic yeast nutrients, it should not be necessary. A properly balanced wort will supply all of the supplementary foods that the yeast requires.

Most brewers' yeasts also require some hop products to be present in the wort for normal behaviour to occur. It is not fully understood why certain strains of yeast require these hop products to be present, or indeed what particular substances it is that they require. It is probably the result of hundreds of years of training our yeast to actually like hopped beer, now it seems that it insists upon it. These hop products are produced when the wort is boiled with the hops and is one reason why some yeasts do not perform well with home-brew kits in which the hop flavours are provided by means of adding hop extracts. These mysterious products are not present in hop extracts.

If you examine the old log for Kingston Ale, you will observe that brewers of old had the habit of adding flour and salt to the ale after the first yeast skimming. This supposedly nourished the yeast; the logic presumably being that yeast has no problem fermenting bread dough, of which flour and salt are the primary ingredients, therefore the same ingredients must be an ideal yeast food. However, the practice, although not exactly misguided, was probably unnecessary.

Sugars (carbohydrates)

Before we proceed any further with this discussion on yeast, it would be as well to include a little background on sugar. Yeast is able to convert sugars into aicohol and carbon dioxide, but sugar is a very general term indeed, scientifically it has a much wider domain than the stuff that we simply stir into our tea. There are many different sugars, or carbohydrates, which can be derived from a variety of materials. There are simple sugars such as fructose and glucose, and there are more complex sugars such as sucrose and maltose; sucrose being cane sugar, and maltose being the sugar derived from our malt. Sugars consist of carbon, hydrogen, and oxygen atoms, and all sugars are classified as to their complexity, and the number of carbon atoms per molecule that they contain.

Monosaccharides

Simple sugars are known as monosaccharides (single molecule sugars). The three monosaccharides that are of interest to us, as far as brewing is concerned, are:

Glucose, Fructose, and Galactose

All three of these simple sugars have the same general formula, (C6H12O6) and differ only in their structure. As can be seen from the formula, these simple sugars contain six carbon atoms per molecule and are thus known as Hexose Monosaccharides. Despite the name being somewhat of a tongue twister, brewers' yeast is able to ferment ail three of these sugars without difficulty. These simple sugars are the basic building blocks of the complex sugars. All complex sugars consist of two or more simple sugars bonded together molecularly.

Disaccharides

Disaccharides are sugars which consist of two monosaccharides with a molecular link, or bond between them. Common disaccharides and their component monosaccharides are:

Sucrose (cane sugar) = Glucose + Fructose

Maltose (malt sugar) = Glucose + Glucose

Lactose (milk sugar) = Glucose + Galactose

These sugars have the general form (Cl2H22O11) because a molecule of water (H20) has been given off in the bonding. Brewers' yeast is unable to ferment disaccharides directly; the molecular bonds, or links, between the two component sugars must be broken first. As far as sucrose and maltose is concerned, the yeast secretes enzymes (invertase and maltase respectively) which break the sugar down into its two component parts. However, the yeast is unable to secrete an enzyme capable of breaking lactose into its component parts, thus lactose (milk sugar) is non fermentable, and can be used for the sweetening of beers.

Polysaccharides and Oligosaccharides

The more complex sugars known as polysaccharides and oligosaccharides consist of a number of simple monosaccharides bonded together molecularly; oligo meaning few, poly meaning many. Starch is in fact a polysaccharide - technically it is a sugar. During mashing, enymes attack the molecular bonds between the starch molecules and break the starch down into simpler sugars, the simpler sugars being maltose, maltotriose (a trisaccharide), and various dextrins. Dextrins are oligosaccharides, but brewers' yeast is not capable of breaking dextrins down completely and in any case is only capable of attacking them very slowly. Dextrin is therefore only partly fermentable, or slowly fermentable and is responsible for the residual sweetness and mouthfeel in beers.

It's the enzymes that do it!

The yeast does not convert the sugars in our wort to alcohol and carbon dioxide directly, but instead release enzymes (sort of biological catalysts) which do the work for it. The group of enzymes responsible for alcoholic fermentation are often called the zymase complex, but that is an old-fashioned name left over from a hundred years ago when brewing scientists thought that only one enzyme (zymase) was responsible for alcoholic fermentation. In fact there are several which work together. Some of these enzymes break down the complex sugars in the wort (maltose) into simpler sugars such as glucose, others work on the simpler sugars.

When the yeast is deprived of dissolved oxygen, it releases these specialist enzymes which are able to break down hexose sugars in the wort into their component parts, other oxidising enzymes then act upon these components to synthesise the energy that the yeast needs to function. The yeast is able to respire without dissolved oxygen because oxygen atoms are one of the component atoms of sugar. Alcohol and carbon dioxide are by-products of this process. The alcohol and carbon dioxide is produced as the result of the yeast and its enzymes changing the molecular structure of the sugars by removing certain components that the yeast needs to produce energy.

The major sugars that are to be found in our wort are maltose (malt sugar), sucrose (cane sugar), and dextrins. The sugars are able to penetrate the cell wall with the aid of a group of enzymes called permeases. Once within the cell maltose is acted upon by an enzyme called maltase; this converts the maltose to glucose, which is readily assimilated by the yeast cell. Sucrose is acted upon by another enzyme called invertase which breaks down the sucrose into invert sugar which is a mixture of glucose and fructose, both of which are readily assimilated by the yeast cell. Some non-fermentable sugars (dextrins) and some residual solids remain.

The dextrins are a group of sugars that brewers' yeast is unable to attack, or is only able to attack very slowly. There are some strains of yeast that are able to ferment dextrins but they are not often used for brewing purposes because they are usually unable to ferment maltose. Some classes of beer (known as Lites) have, in fact, had some of the dextrins fermented down along with the other sugars, but this is usually achieved by adding a special bacterium or an industrial enzyme to the wort during fermentation. This enzyme (amyloglucosidase) converts dextrins into glucose. The quantity of dextrins in our wort is determined by our grist make-up and by the mashing stage of our beer production.

Once the sugars have been converted into a suitable form and are within the yeast cell, the oxidising enzymes begin work and break down the sugars into alcohol and carbon dioxide, and in so doing release energy for use by the yeast cell. Not all of the energy thus released is used by the yeast cell. The surplus energy causes the temperature of the wort to rise during fermentation, although this effect is not apparent in the small volumes used in home brewing.

Fermentation by-products

Besides producing alcohol and carbon dioxide there are many other compounds that are produced in small amounts by the fermentation process. It is these by-products of fermentation that are responsible for much of the taste, character and aroma of your favourite pint, and are often referred to as flavour compounds. Some of these compounds are hydrocarbons and higher alcohols which go under the general name of fusel oils, other by-products are esters and organic acids. Different strains of yeast produce different fermentation by-products in varying quantities.

Furthermore, the type and the relative quantities of these important compounds are affected by wort composition, fermentation temperature, and wort acidity (pH). Even the dimensions of the fermentation vessel are acid to affect them! It is for this reason that one brewery may find it impossible to duphcate a beer from another, even though they may be using exactly the same recipe. It is also why a brewery places so much importance upon its strain of yeast.

Some of the by-products of fermentation, such as methyl alcohol and acetaldehyde are undesirable in beer, but these disappear during the maturing stage. A wort that has been fermented at a too high temperature will usually produce a beer that has unpleasent off-tastes. This is due to the fact that different temperatures and pH will activate different enzymes, which in turn produce different by-products, some of which have unpleasant tastes and do not disappear during maturation.

Adaptability

Brewers' yeast is extremely adaptable to the changing conditions in which it finds itself. One assumes that given the rapid speed at which it multiplies, minor evolutionary changes in its make-up are achieved relatively quickly. The practice of brewers pitching one brew with yeast taken from a previous brew is auto-selective and considerably aids the evolutionary process. This preferentially selects yeast which performs well in the type of beer being produced and which has adapted to the minerals and other nutrients that the water supply or wort composition provides. Yeasts that do not have sufficient alcohol tolerance, wild yeasts, or mutant strains, either die off, sink to the bottom or remain in suspension in the wort. Either way, these unwanted yeasts are not skimmed off, and therefore are not propagated.

Yeast does not like sudden changes in its environment; it can go into osmotic shock. A yeast that has spent the last few generations producing a light mild ale may object most strongly if it is suddenly plunged into the wort of a strong old ale. The yeast's cell shape, wall thickness, and make-up gradually change to take best advantage of the nutrients and minerals that it finds in a particular wort. Sudden changes causes it problems. Sudden changes in temperature can also throw the yeast into osmotic shock.

Dried yeasts

The vast majority of home brewers use the packaged dried yeasts supplied by the home-brew trade, and this is by far the easiest way of obtaining yeast. However, the "authenticity" of some packaged yeasts is questionable, and some care should be exercised when purchasing them. It is important to always buy high quality branded yeast that states quite clearly on the packet that it is suitable for the type of beer that you intend to brew. Never buy a yeast on the basis of cost alone; for the sake of a few coppers it is not worth risking a brew.

Be wary of yeasts which are labelled in a non-committal manner. The terms beer yeast, brewers' yeast, or even genuine brewers' yeast can mean anything. It is certainly not clear from the description whether it is supposed to be an ale yeast or a lager yeast. These are often cheap, general-purpose yeasts which are neither one thing nor the other, and although they may "work" so to speak, they will not provide true-to-type charcteristics in the finished beer. The anonymous little packets that come with some of the cheaper beer kits should be thrown away for the same reason. Unfortunately the beer-kit market is price-driven as opposed to quality-driven, and the yeast supplied with some of the kits is of very dubious origin.

By definition, Ales are brewed witn a top-working strain of yeast. When buying an ale yeast ensure that the packet states that that is what it is. A Packet labelled with a description such as: ale, real ale, English ale, or even "English beer"is rather more specific than a non-committal brewers' yeast, and is more likely to contain a top-working strain of Yeast that is suitable for brewing ale.

The situation is easier for bottom-workers; the labelling is much more specific. Yeasts labelled Munich beer, Carlsberg yeast, Danish beer, and simply lager yeast are all available via the home brew trade, and we can safely assume that these are true-to-type bottom-workers. However, I must confess that one supplier's labelling Danish Ale leaves me somewhat besumed.

Most packets contain only five grams ofyeast. This, in rny opinion, is nowherenear enough to ensure a reliable start to a five gallon brew. Most commercial breweries pitch with twenty times this rate, and with an active, live yeast at that, not a dried imitation. The unstructions on packets usually instruct the brewer to mix the yeast into a slurry with a little lukewarm water or cold beer and then add it to the brew. With a high-quality, active and viable yeast this technique may be okay; indeed many perfect brews have been produced in this way. However, yeast is fickle stuff and I would advise always making up a yeast starter bottle before brewing. This way you can ensure that the yeast is willing to perform for you before commiting it to a five-gallon brew. The mere sprinkling of a few grains of a dried yeast on to the surface of the wort is risky.

If for any reason you are unable to make up a yeast starter, the use of more than one packet of yeast will provide a higher level of certainty.

Yeasts in liquid suspension

Yeast can be obtained in liquid suspension form. This is supplied in a vial containing a mixture of yeast and a fluid that is intended to keep the yeast in suspended animation. Liquid suspensions would seem to offer many advantages over dried yeasts, not least the fact that there is no drying process employed, thereby providing yeast of potentially higher viability. Another advantage is that it would be easier to supply some of the more specialist strains of yeast which would otherwise be unobtainable due to the economics of drying and packaging small quantities.

Yeast in suspension form is available from some suppliers, but it seems to have fallen out of fashion. However, there are difficulties with it Unlike dried yeast it does not have an unlimited shelf-life and it is difficult to keep the yeast infection free. It should, ideally, be kept refrigerated at about 4°C; but many home-brew retailers, particularly the large chain stores, did not keep it refrigerated and quality suffered. Anyway, perhaps it will make a come-back one day.

Real live brewers' yeast

In the past, some home-brew shops sold tubs of fresh live brewers' yeast which they obtained from a local brewery and packaged it themselves, usually in those polystyrene coffee cups that are fitted with lids. Other shops expected you to take your own container. Unfortunately this is a service which seems to have disappeared, although many commercial micro-breweries have a brewery shop on their premises and some are prepared to sell their yeast to home brewers.

If you are lucky enough to have access to live brewers' yeast you will find a phenomenal difference in performance between this and packaged yeast. Real live brewers' yeast will perform like a controlled nuclear explosion when compared to the dried stuff. A quantity of 20-50 grams is more than sufficient to start a five-gallon brew. A yeast starter is not required under these circumstances. Simply light the blue touch paper and retire immediately!

if you have access to yeast from a brewery, you will probably be expected to take your own container. By far the best way of transporting live, wet yeast is in a domestic vacuum flask of the Thermos variety. Before collecting your yeast, dean and sterilize your flask with Chempro SDP and rinse thoroughly. Fill the flask with crushed ice, or (very carefully) with ice cubes, and seal the flask. When you reach the brewery, or home-brew shop, tip out the ice, put in the yeast and re-seal. This provides a cool insulated environment for the yeast's journey and it will respect you for it. If you are not ready to use the yeast immediately you get home, place the yeast, flask and all, into the refrigerator - not the freezer. Don't risk contamination by transferring it to a different container.

However, because you have obtained your yeast from a brewery it does not follow that it is ideal tor home brewing. Some breweries' strains, particularly those of northern breweries, are quite temperamental and some need to be roused at regular intervals. The breweries tolerate this situation because their particular yeast imparts a flavour which they desire. As home brewers we normally do not have the time to continually watch over our beer, therefore, a yeast that requires frequent rousing is undesirable.

Yeast starter culture

I consider it to be an essential prerequisite to any home-brewing session to make up a yeast starter solution. The average packaged yeast in Britain contains about five grams, which is not really enough to ensure a rapid start to the fermentation of a twenty-five litre brew. It is essential that our beer is supplied with a adequate quantity of active yeast to ensure that it is able rapidly to establish itself and form a protective head before bacteria have a chance to gain a foothold.

The time between pitching the yeast into our wort, to the yeast multiplying sufficiently to provide a protective head and begin fermentation, is a dangerous period of our home-brewing process. Brewers' wort is a perfect culture medium, rich in all sorts of nutrients, vitamins and minerals. All manner of little "nasties" are quite happy to breed in it, and will do so if they are given the opportunity. If a "nasty" succeeds in establishing itself before the yeast does, then the chances are that the beer will be ruined.

Commercial breweries pitch with about one-and-a-half pounds of yeast per barrel, which equates to about 100 grams per twenty-five litre home-brew batch. Although we do not necessarily require as much as 100 grams - that is probably a bit over the top - we do require a lot more than five grams if a reliable start is to be achieved.

A yeast starter solution takes only a few minutes to make up. It is made a couple days before we plan to brew and ensures that we have an adequate quantity of active yeast grown under sterile conditions before we begin. We can confirm that the yeast is viable, active and bacteriologically sound before committing it to a twenty-five litre batch. This minimises the risk of later problems with yeast performance or infection. The lag phase, i.e., the time between pitching the yeast and something starting to happen, is reduced considerably because the yeast has converted to aerobic respiration and is actively multiplying before it is pitched.

It is important to ensure that a yeast starter does not inadvertently become a bacteria starter as well. Therefore it is essential that a starter is prepared under conditions of the greatest sterility that we are able to achieve. A combination of chemical sterilisation and heat is the easiest way of ensuring a sterile medium. I use a standard, "dumpy" style, one-pint milk bottle as a culturing flask because it is designed to withstand boiling water and it will accept a wine-maker's airlock and rubber bung. So far I have had no accidents caused by transferring boiling fluids to milk bottles, but the possibility exists that the milk bottle may shatter. Please exercise the utmost caution when messing around with boiling fluids.

Making a yeast starter is straightforward enough:

1. Sterilize a standard one-Pint milk bottle, an air lock, a rubber bung, and a small funnel using domestic bleach, CnemPro SDP or similar. After sterilization rinse the items very thoroughly. Pre-heat the milk bottle and fill it with boiling water from the kettle and leave it to stand.

2. Bring approximately half a pint of water to the boil in a saucepan and then add four generous tablespoons of malt extract (50-60 grams). Simmer for about a minute, stirring continuously to avoid burning, and then turn off the heat.

3. Carefully empty the hot water from the milk bottle and then pour in the hot malt extract solution via the funnel. The bottle should be about half full. Loosely cover the mouth of the bottle with a piece of aluminium kitchen foil and stand it in a bowl of cold water until it has cooled to room temperature. The solution will cool faster if the bottle is given a shake from time to time in order to re-distribute the contents.

4. When the solution has cooled, and with the foil still covering the mouth of the bottle, give it a vigorous shake to admit oxygen into the solution, taking care not to discharge the contents all over the kitchen.

5. When you are sure that the solution has cooled to room temperature, add your yeast, give the bottle a final vigorous shake, fit the sterilized bung and air lock, and stand the starter in a warmish place of about 20°C. Ensure that the outside of the bottle is clean and free from malt extract solution as this will be a potential bug trap.

Do not brew your main batch of beer until you are sure that the yeast is actively fermenting. This will be indicated by the typical frothy head developing and carbon dioxide bubbles propagating through the airlock. The time taken for the starter to become sufficiently active for pitching is dependent upon yeast viability, initial quantity of yeast, and temperature. Anything from a few hours to a couple of days can be experienced. If you intend to brew on a Sunday morning you would probably make your yeast starter on a Friday night; it can be kept under airlock for a couple of days until you are ready to brew. Give the starter a sniff before using it to ensure that it smells okay and is therefore likely to be infection-free.

If you are fortunate enough to have access to a Pyrex laboratory flask, a more sterile method of producing a yeast starter medium is to add the water and malt extract to the flask, and heat the flask directly, simmering the contents for about a minute. Cover the mouth of the flask as above, and cool the flask under cold running water. When the flask has cooled to room temperature, add the yeast, shake, fit airlock etc.

You may be tempted to put the malt extract and cold water into a milk bottle and boil it in one hit in the microwave oven. At first thought this seems an ideal way of achieving ultimate sterility, but apparently it is a very dangerous practice. Boiling in the microwave generates super-heated zones in the liquid which can cause the contents of the bottle to erupt like a volcano as soon as it is disturbed, with disastrous consequences. Sticky super-heated fluids sprayed over one's person cannot be much fun, so please don't do it.

The mouth size of milk bottles seem to vary around the country. Some accept a one-gallon demijohn bung, while others only accept the half-gallon bung which is more difficult to obtain. An alternative is simply to plug the mouth of the bottle with cotton wool. Clean the bottle in Chempro SDP and rinse thoroughly before returning it to your milkman. Dried malt extract is available from your home-brew shop and is more convenient than ordinary malt extract syrup when used for this application. Dried malt extract should be stored very carefully and in a dry place because it will absorb moisture from the atmosphere and set in a hard lump, like toffee. I store mine in an air-tight sandwich box. If your dried malt extract does happen to set hard simply break it up with a suitably sized whacking implement. It will still be perfectly suitable for yeast propagation purposes.

Recovering yeast from a bottled beer

The ability to recover yeast from a bottled beer provides a useful method of obtaining obscure yeasts, or yeasts appropriate to a particular style of beer. It also enables the home brewer to propagate yeast by recovering yeast from his own bottled beers.

Most British bottled beers are filtered or pasteurised these days which removes or kills the yeast. However, some breweries produce a live, bottle-conditioned beer and there is an increasing number of imported speciality beers which are unpasteurised.

English home brewers have traditionally used yeast kidnapped from a Guinness bottle. However, Guinness has withdrawn its bottle-conditioned stout, so this option has disappeared. Bottled Worthington White Shield is always unpasteurized no matter where you buy it, so this seemed to be the surest way of obtaining brewers' yeast. Unfortunately, Bass have recently changed the yeast strain in Worthington White Shield, and is now reputed to be a bottom-worker.

However, an increasing number of commercial breweries are producing bottle-conditioned beers. Perusal of the CAMRA Good Beer Guide will enable you to identify candidates for yeast supplies. Among the better known are Courage Imperial Russian Stout, Eldridge Pope Thomas Hardy Ale, Gales Prize Old Ale, Burton Bridge Burton porter, King & Barnes Festive, Shepherd Neame Spitfire. All are supplied in bottle-conditioned form.

The procedure for propagating yeast from a bottled beer is quite simple.

1. Stand the donor bottle in a cool place and leave it undisturbed for a day or two to allow the yeast to settle to the bottom.

2. Make up a yeast starter using stages 1 to 4 of the procedure described above.

3. When the starter has cooled to room temperature, carefully uncap the donor bottle and decant all but the last half-inch of the contents into a glass, taking care to ensure that any yeast sediment is left behind in the bottle.

4. Give the remaining contents of the donor bottle a good shake in order to dislodge any yeast clinging to the bottom of the bottle. Tip the entire contents, dregs and all, into the yeast starter solution using the funnel. Shake, fit a sterilised airlock, stand in a warm place etc, as above.

5. Drink contents of glass.

Some commercial beers have a very low residual yeast count and it may take several days for signs of activity to show in the starter medium. Some types of yeast pack firmly down on to the bottom of the bottle. With these types of yeasts it may be necessary to pour the solution from the starter into the donor bottle, shake and pour back into the starter repeatedly until the yeast has been dislodged.

Propagating yeast

Many home brewers, once they have found a yeast which suits them, will want to continue to use it. If this happens to be a packaged yeast there is no problem unless the packager changes his source of supply or withdraws the product. However, if the yeast is from a commercial brewery, or from some other obscure source, the home brewer will need to propagate the strain himself. Many home brewers propagate their own yeast because propagation has the advantage of adapting the yeast to perform well in their particular style of beer. Another advantage is that the yeast supply is of known performance and origin, and free from change imposed by commercial, packagers.

One of the earliest methods of yeast propagation that I have come across probably dates back to the earliest ages of home brewing in this country. With this quaint method the housewife would make up a small faggot of birch twigs, rather like a miniature witch's broom, and float this on the surface of the fermenting ale. When fermentation was complete the faggot was removed from the ale and hung up to dry. When the next batch of ale was made this faggot was again floated on the surface of the wort. The film of yeast which covered the twigs would start the new batch fermenting, after which the faggot would be removed and hung up to dry again. And so on. Cunning eh? But, unfortunately, this is not suitable for our more modern times.

Another old method of yeast propagation used by the housewife was to make yeast cakes. A year's supply of these were made during the summer and were baked in the sun. These apparently went as hard as ship's biscuit, which, according to William Cobbett, is much harder than the timber of Scotch firs. The fact that these were baked at low temperatures in the sun, as opposed to in an oven, preserved the yeast. Being "as hard as ships biscuit" they would keep almost indefinitely provided that they remained dry. The evening before the housewife was due to bake or brew, she would break up one or more of these yeast cakes, dissolve the crumbs in a little warm water, and allow this to stand overnight in a warm place. It was then added to the bread dough or ale wort to start fermentation. In those days ale yeast was used for baking as well as brewing.

Commercial breweries have traditionally propagated yeast by saving some from the top of one brew to start the next. However, commercial brewers have the advantage of brewing almost daily therefore yeast collected in this way does not hang around long before it is re-pitched, and is unlikely to go off Amateur brewers do not usually brew frequently enough for this to be a practical proposition, but if one is brewing several batches of beer in quick succession some yeast can be saved from the top of a brew and stored on a short-term basis.

Skimming yeast for storage

The first crop of yeast that forms on the surface of the wort is not suitable for re-pitching. The majority of this first crop has multiplied by utilising the dissolved oxygen in the wort (aerobic respiration), and has not even begun to think about producing alcohol. If we continually propagated our yeast using this crop we could end up with a variety of yeast that disliked alcohol! Furthermore, this first crop contains a high proportion of mutant cells, wild yeasts, bacteria, and it also contains solid particles and other non-soluble matter carried up from the wort by the rising yeast. Likewise, the yeast sediment on the bottom of the fermentation vessel is also unsuitable because this again contains mutant strains, bottom working varieties, dead cells, and yeast that, for one reason or another, has been unable to go all the way.

It is best to wait until the yeast has firmly established itself. Scrape off the dirty top surface of the yeast to reveal the fresh clean stuff close to the surface of the wort and use this. Alternatively, if you use the dropping system described in the chapter on fermentation, take the yeast from the new head that is formed after dropping.

Storing skimmed yeast

Yeast skimmed from one brew can be kept in a refrigerator for a short term, say about a week but this method is not really suitable for longer periods of time.

To store skimmed yeast in the fridge, remove as much beer residue as possible by squeezing the yeast between filter paper, blotting paper or, at a push, paper kitchen towel. Put the yeast into a polythene bag and fold it tightly to exclude air, place it into an air-tight container and store it in the main compartment of the refrigerator (not the freezer). Optimum storage temperature is 4°C. This will keep for about a week, but not much longer.

A simple suspension

Longer-term storage can be achieved by storing the yeast in a twenty per cent sucrose solution. A twenty per cent sucrose solution will have a specific gravity of about 1080, and can be made up by taking twenty grams of household sugar and adding de-ionised or distilled water to make the volume up to 100 ml. The yeast is added to this and then a small quantity of paraffin or other mineral oil added. This will keep for a couple of months or so in an air-tight container at 4°C.

The sugar retards osmosis and places the yeast in suspended animation. The use of pure cane sugar and de-ionised water ensures that there is a lack of nutrients for the yeast to feed upon, thereby retarding yeast activity. The mineral oil will float on the surface of the suspension and ensure that anaerobic conditions are maintained. The sugar should be boiled into solution to ensure sterility. As much beer residue as possible should be removed from the yeast as this is a potential source of nutrients.

A professional or commercial suspension would use mannitol (sugar alcohol) or sorbitol (a fairly inert form of sugar) as the osmotic retarder. They would also contain about 80 ppm of potassium metabisulphite and perhaps some citric acid to keep bacteria at bay.

When re-using the yeast, the yeast is removed by inserting a syringe or pipette below the surface of the oil and drawing the yeast into the syringe, leaving the oil behind. Alternatively the oil can be removed from the surface with a syringe or decanted off the solution. The yeast should be re-cultured using the starter medium described above. Syringes can be obtained from chemists and some home-brew shops stock them.

Freezing yeast

Yeast is difficult to freeze successfully because the ice expands and ruptures the cells; usually only the spores survive. However, viability can be greatly improved by the addition of glycerol as an antifreeze. If you want to store yeast in the freezer, make up a suspension medium as described above and add glycerol to the equivalent of about fifteen per cent of the total quantity of fluid. This will keep in a freezer for up to a year with luck.

Glycerol can be obtained from a chemist. Home-brew shops sell glycol-based wine sweeteners, usually propylene glycol, or similar. This can be used as a substitute for glycerol and used in the same way. Some home-brew shops sell glycerol for some obscure winemaking purpose.

Re-pitching yeast for long-term storage should always come from the strongest beers that you produce. Never re-use a yeast that has come from a brew that has been spoiled for any reason, or a yeast that looks discoloured or smells strange.

Long term yeast propagation

The most successful and probably the best amateur method of long-term yeast propagation is to propagate yeast from the dregs of home-brewed bottled beers. It is best to brew a special batch of very strong ale specifically for yeast propagation purposes, and bottle it. The yeast will remain dormant in a bottle of sound strong ale for several years. The yeast can then be re-cultured from these bottles as and when required. Yeast obtained in this manner is of good parentage, its ancestry goes right back to pitching, and it has come all the way down the line to producing good beer. It also offers the advantages of having a pint of beer to drink while one is busy yeast-propagating, and having a stock of strong mature ale for special occasions. The best type of beer to brew for propagation purposes is a strong, very hoppy brew of OG 1065 or greater; the higher the gravity the better. The high alcohol and hop content will help to keep the beer sound. Take extra care over sterilization and cleanliness for this particular brew. If the beer goes off in storage you cannot re-use the yeast. It is best to brew this particular batch during the winter when it is much less likely to become infected with airborne bacteria. After the bottles have been filled and capped, wash the outside of them with Chempro SDP or house-hold bleach to remove any traces of spilt beer which could serve as potential bug traps. When you wish to re-propagate the yeast it can be recovered from the bottles using the techniques described above for kidnapping yeast from commercial beers. You can, of course, recover yeast from your ordinary weaker bottled beers at intermediate times, but the special brew should be regarded as your primary source, and you should never allow yourself to run out of it. Always ensure that the beer is sound by smelling and tasting it before re-propagating the yeast.

Ale-yeast performance

A good, top-working, English Ale yeast should quickly establish itself into a thick, dense, rocky head. This head should be two or three inches deep, have a density of something like whipped cream, and look like something out of Quatermass. The term "rocky head" is an attempt to convey the impression of a thick, ragged, uneven surface of chunky appearance as opposed to a weak, self-levelling foam; although it may settle into a level mass towards the end of fermenta-tion. It is difficult to describe or illustrate the effect precisely because the appearance changes with differing conditions and during different stages of fermentation; indeed every head is different, and could be considered an art form. However, the description should be adequate to give you the general idea of things and if you achieve something close to it your yeast is per-forming well.

A good yeast should contribute no undesirable flavours to the beer, should rapidly form a thick protective head during fermentation, and should remain vigorous at temperatures down to about 18 C (60 F).

Other requirements of a good yeast is that it clears down quickly and unaided after barrelling. If you bottle your beers, a type of yeast that packs down firmly and adheres to the bottom of the bottle as a film would be an advantage. Guinness yeast behaved in this manner, which is why many home brewers used it. Some other yeasts collect as a loose clump at the bottom of the bottle and require more care when pouring.

Types of ale yeast

English ale by definition uses a top-working yeast, but there are many strains of top-working yeast and the behaviour of individual strains vary considerably. However, there is a distinct different between the style of yeast traditionally used by commercial breweries in the north of

England, and that used by commercial breweries in the south of England. Both styles are top-working ales yeasts, but they differ considerably in their characteristics. It is therefore possible to classify English ale yeasts into two broad sub-divisions, which I have euphemistically termed "northern yeasts" and "southern yeasts", although the geographical distinction is not particularly apparent these days.

Northern ale yeasts

Northern yeasts produce beers that are distinctly full and carry a rough sweetness on the palate that is preferred in northern countries. They are common in the north of England, Yorkshire in particular, but can also be found in Manchester and other areas. Northern yeasts do not particularly enjoy making alcohol and need frequent rousing to maintain fermentation. That is, every three hours or so the yeast head needs to be stirred back into the wort. Some strains require frequent aeration as well, and as a consequence they produce copious amounts of surplus yeast which needs to be constantly skimmed off. Northern yeasts do not normally attenuate so deeply as their southern counterparts. That is, they generally produce less alcohol for a given gravity beer.

A unique fermentation vessel known as a "Yorkshire Stone Square", now almost extinct, evolved to cope with this style of yeast. A Yorkshire Square needs attention every two hours. Modern breweries that use northern yeast keep the yeast active by continuously pumping the wort from the bottom of the fermentation vessel and spraying it back in at the top. This keeps the wort circulating through the yeast head, which is effectively what a Yorkshire Stone Square did. The risk of infection from airborne bacteria is high. Northern style yeasts would be quite troublesome in the home-brew environment. We normally would neither have the time to attend to our fermentation every few hours for five or six days, nor the skills required to build special fermentation equipment to cope. A home brewer would be best advised to stick to southern-style yeasts, irrespective of the style of beer that he is intending to brew.

Southern ale yeasts

Southern yeasts yield beers that are clean and dry - flavours that are favoured in the south. Southern style yeasts are better adapted for use in ordinary open vessels than their northern counterparts. They are much less troublesome and can be left to get on with their work unattended. They should not need rousing and should not need re-aeration under normal circumstances. They exhibit greater attenuation, produce more alcohol than their northern counterparts, and provide richer fermentation by-products. The English Ale yeasts supplied to the home-brew trade would be southern style yeast, often Whitbread strain B, which is quite a vigorous hard worker.

Lager yeast

Most of the comments in this chapter regarding ale yeast can be applied equally to bottom-workers; usually referred to as lager yeast in the home-brew field. However, there are some characteristics peculiar to bottom-workers, and this section is intended to highlight those.

Genuine Pilsner-style lagers incorporate a lagering process. This is a low temperature secondary fermentation, or maturation stage, performed by storing the beer for up to six months at a temperature of around 4°C. A special low temperature, bottom-fermenting yeast strain is employed for this lagering stage. With some lagers the yeast used for primary fermentation is not necessarily the same as the one used for the lagering stage; the yeast is filtered out after primary fermentation and replaced with another strain for secondary fermentation. In a few cases a top-working yeast is used for primary fermentation, particularly if the brewery concerned uses open primary fermenting vessels, but the resulting beer can still regarded as a genuine lager if a low temperature, secondary fermentation stage is employed using the appropriate bottom-working yeast. It is difficult for the home brewer to filter out one yeast and replace it with another, so home-brewed lagers usually use the same strain for both primary and secondary fermentation.

A wide range of packaged lager yeast is available from home brew shops. The labelling on most examples is quite specific, but any yeast that calls itself "lager" can be assumed to be a bottom worker. A true lager yeast, suitable for the lagering stage of beer production, should be capable of working at low temperatures and should produce appropriate flavour characteristics in the finished beer. True bottom-workers are apparently capable ot fermenting the sugar melibose, whereas top-workers are not. However, that knowledge is of little practical use to us. Bottom-working yeasts usually demonstrate top-working characteristics during primary fermentation in the home-brew environment particularly if the fermentation temperature is higher than is typical for lager. The higher the temperature the more top-working the yeast behaves. This is not really a problem. A yeast head is an advantage when brewing in open vessels; it protects the beer from airborne bacteria.

It is not usual with lager to save yeast from one brew to start the next. Being bottom-working the yeast is contaminated with all sorts of other rubbish that settles on to the bottom of the vessel. Taking yeast from the surface is not a good idea because it could train the yeast to be a top-worker. Commercial lager breweries use a fresh laboratory culture for every brew and amateur brewers should do the same. However, it is safe to re-culture yeast from the sediment in bottles should the need arise, and, of course, to steal yeast from the bottles of commercial lagers providing that they are unpasteurized and unfiltered.

Yeast + enzyme

A recent development in home brewing is the availability of yeast with an industrial enzyme added. With some of these the enzyme is mixed with yeast in a single sachet, whereas with others the enzyme is supplied separately. The purpose of the enzyme, amyloglucosidase, is to break down some of the dextrins in the wort into glucose, and thereby provide a beer of higher fermentability, and correspondingly increased dryness. To use an old advertising cliche, it converts more of the sugars into alcohol.

The enzyme enables a type of beer known as "lite" to be produced by the amateur. These are dry, low carbohydrate beers which are higher in alcohol than similar beers because a proportion of the residual dextrins have been fermented out. With a conventional beer the dextrins remain to provide residual sweetness and body, whereas with a "lite" some of these have been turned into alcohol. The use of this enzyme enables fermentabilities in excess OF 80 per cent to be achieved, as opposed to the more usual 62-65 per cent achieved with conventional beers This will produce a characteristically dry beer.

Unfortunately, the packagers have chosen to call these Pilsner yeasts or Pilsner enzymes, which they most certainly are not. This sort of labelling, apart from being illegal, is bound to cause confusion with the genuine Pilsen yeasts from the Czech Republic. In fact, true Pilsen yeast exhibits no greater degree of fermentability than any other yeast, so the concept is mis-guided anyway. Furthermore, a true Pilsen lager does not use enzymes! This misrepresentation is further compounded by the fact that, in the sample I analysed, the actual yeast strain supplied was Whitbread strain B - an English top-worker - hardly Czech! The accompanying enzyme comes from Stockport in Cheshire! However, that is not a problem, Whitbread strain B is used because it Is better suited to our open fermentation techniques than is a true lager yeast. It is the terminology that I object to - not the product. Admittedly, the terms Pilsener, Pilsner, and Pils are the most abused beer terms in the world, but there is no need tor the home-brew industry to compound this.

No doubt the enzyme amyloglucosidase will appear on the home-brew shop shelves pack-aged as a separate item during the currency of this book. Perhaps other enzymes systems will appear as well. Amyloglucosidase should be used as directed on the package, but it is more efficient when added to the fermenting beer towards the end of fermentation rather than at the beginning.

Bacterial contamination of yeast

One of the problems of propagating yeast by saving some from one brew to pitch a succeeding brew is that bacteria can be transferred from brew to brew along with the yeast. Fortunately the home-brew environment is such that, providing reasonable care is taken, catastrophic bacterial or wild yeast contamination of a blew is rare. Many home brewers have propagated the same yeast strain for many years without taking precautions against bacterial infection, and without suffering any noticeable contamination. Should a yeast strain become contaminated, it is best to obtain a fresh charge from your usual source.

Techniques for cleansing yeast are based upon the fact that brewers' yeasts are tolerant to high concentrations of alcohol, hops, acids, and sulphur dioxide whereas the vast majority of wild yeasts, and bacteria are killed by one or more of these substances. The technique of propagating yeast from speciaally brewed bottled beers, as described above, automatically ensures a high level of sterility. The high level of alcohol and hop products helps to keep the beer sound.

Add washing

Commercial breweries purify their yeast by acid washing. The yeast is mixed into a slurry with water and then phosphoric acid, or sulphuric acid is carefully added until the pH of the solution is about pH2-pH2.5. The solution is allowed to stand for four or five hours. After the standing period the yeast is removed from the solution by means of a filter paper and immediately re-propagated. It is nothing short of amazing that yeast survives this treatment, but it does. The acids concerned are nasty corrosive substances, therefore this approach is unsuitable for the amateur brewer.

Yeast weakness

Yeast weakness is a euphemistic term mat is employed in the brewing world to describe a common condition whereby the yeast suddenly loses its power to ferment the wort ie. the fermentation velocity or the fermentation efficiency suddenly decreases tor no apparent reason. Yeast weakness covers a multitude of different ailments. It is difficult for the home brewer to determine which of the different possibilities is the real cause of the problem, and even more difficult to do anything about it should the cause be known.

Yeast weakness can be caused by genetic mutation in the yeast strain. It can be caused by a wild yeast straying into the colony and either becoming established or changing the genetic make-up of the bulk. A further problem with wild yeast contamination is that certain strains of yeast produce components called zymocides which serve as poisons to other strains of yeast and retard their fermenting powers or even kill them off completely. It is now known that certain viruses can attack yeast and actually make it ill. Sometimes a yeast can lose its ability to respire anaerobically, but still be able to respire aerobically. Of course the problem may not be due to a yeast weakness at all. It could be caused by incorrect fermentation temperature, acidity, or nutrient deficiency. It is also possible, although unlikely if the normal precautions are taken, that a bacteria has succeeded in establishing itself to the detriment of the yeast. If your yeast is apparently suffering from a yeast weakness try rousing the yeast, it that tails re-oxygenate the wort by pouring it from one container to another, if that fails throw the brew away and obtain a fresh change of yeast. Take comfort in the fact that yeast weakness sometimes plagues the largest and most scientific commercial breweries, and their brews are accordingly thrown away, or, more likely, pasteurised and blended back into other beers.

Yeast frets (Cask frets)

When transferring beer into barrels or bottles it is important that precautions are taken to ensure that a minimum of air gets into solution. Not only will dissolved air enable unwanted aerobic bacteria to grow, but if the yeast finds a plentiful supply of air it will revert to aerobic respiration and begin to multiply sifinificantly. Not only will this produce an excessive quantity of yeast in the beer, but when the air is used up the yeast will try to adapt to anaerobic respiration again, but it may find a lack of nutrients on which to work and become stuck in a transitionary phase.

Under these conditions the yeast will remain in suspension and could take a long time to clear down properly.

This condition can also be caused by an excessive amount of air in the head space of casks. ideally a cask should be quite full when it is filled, except, perhaps for a little bit of space to allow for liquid expansion with temperature. Not only does a full cask leave insufficient room for a significant amount of air, but the pressure build-up is more reliable because there is no ullage to compress. The pressure build up in the cask aids the clearing process.

If your cask has a significant amount of empty space it may be beneficial at the time of filling to add some cane sugar primings to build the CO, pressure up quickly. After a couple of days, expel air by releasing the cap, or lifting the valve slightly, and then re-seal. Repeat this a couple of days later, and then leave the ale in peace to mature.

I am sure that the bad reputation that home brew has gained for being yeasty and turbid is partly due to yeast frets caused by having too much air in solution, or in the head-space of casks and bottles.

Comments

0#1Stuart Roberts2011-09-22 01:56Hito the best of my knowledge all strains of Saccharomyces cerevisae used in commercial brewing are polyploidy - that is instead of having 2 sets of chromosomes, they have 3, 5 or even 7. Just as children with Down's syndrome can grow and heal themselves when cut, but because they have an extra chromosome - 47 instead of 46 - they cannot produce fertile eggs or sperm, so brewers' yeasts with an odd number of chromosomes, can only reproduce by budding, they cannot form spores.Quote